1. Field of the Invention
The present invention relates to a digital filter device which performs a digital filtering process.
2. Description of the Related Art
In applying a digital filter to an electronic musical instrument or the like, it is desirable that the filter characteristic can be dynamically changed. A cutoff frequency is a parameter that indicates the boundary between the frequency passing region and the frequency cutting region of a filter. Altering the cutoff frequency of the filter can change the tone quality and timbre of sounds produced by an electronic musical instrument. It is possible to directly compute individual coefficients (filter coefficients) of a transmittance (transfer function) of a digital filter from the cutoff frequency. However, this computation needs calculation of trigonometric functions, etc., thus requiring a considerable amount of time. The computation of the filter coefficients inevitably puts a great load on a control device (CPU) which should perform other processing. As a solution to this shortcoming, therefore, a conventional system includes a memory (filter coefficient conversion table) to store filter coefficients corresponding to the individual cutoff frequencies, and accesses this conversion memory with a specific cutoff frequency to thereby acquire the corresponding filter coefficient. The memory should, however, have a large memory capacity to store the filter coefficients for all the specific values of the cutoff frequency.
In applying a digital filter to processing musical tones, it is desirable that the resonance index as well as the cutoff frequency in the filter characteristic can variably be controlled. If the aforementioned conversion memory is used to acquire a filter coefficient from variable, specific values of the cutoff frequency and resonance index, the required memory capacity would become significantly large in proportion to (the total number of the cutoff frequencies).times.(the total number of resonance indexes).